This invention relates to a valve, and more particularly to a valve for service in a high-pressure, high-temperature fluid system, such as in urea synthesis or similar processes.
Heretofore, valves used in high-temperature, high-pressure processes were typically short-lived, often did not properly seal, and were expensive. Generally, these prior art valves were high-pressure split globe valves with a stationary seat in the valve body and a valve member movable between an open position and a closed position by turning a handwheel and threaded screw arrangement. These prior art valves required as many as about 40 turns to fully open or close the valve and the valve operator had to exert relatively high turning torque on the handwheel to open and close the valve. When closed, sealing was accomplished by a metal face-to-metal face seal between the valve member and a stationary seat.
In order to make a leak-tight face-to-face seal it is critical that the movable valve member be maintained in precise alignment with the stationary seat and that both of the seats be clear of any obstruction, such as dirt, corrosion, or solidified system fluid which would prevent the valve member from properly seating on the stationary seat. It has been found that small leaks in a face-to-face seal will, in a relatively short time, cause deterioration of the valve and valve member when the valve is subjected to high-system fluid pressure. More particularly, a fluid under high pressure will flow through a small leak in a face-to-face seal at very high velocity and this fast-moving fluid will rapidly erode the valve member and the valve seat. This type of erosion is commonly referred to as wire drawing. Of course, as the valve seat or valve member is worn away, the leak rate of the valve will increase.
In certain manufacturing processes, such as in the manufacture of urea, additional problems are encountered by the system valves. For example, the system fluid in a conventional urea process must not only by maintained under high pressure, but its temperature must be maintained above the freezing point of urea (approximately 130.degree. F. or more) so as to prevent it from freezing in the system. For example, the system fluid in a urea synthesis process is typically maintained at 2500 psi and at a temperature of about 400.degree. F. To prevent undue cooling of the system fluid as it flows through the valve, valves are often heated as, for example, by wrapping the valve with steam lines and circulating steam therethrough. In practice, however, these added-on steam lines have not always adequately heated the valve and the steam lines themselves have caused many maintenance problems. In the event, the system fluid would freeze in known prior art valves the solidified fluid would prevent the valve from moving from its closed to its open position. This, of course, could cause a serious risk to the safety of the process. In the event a prior art valve would freeze closed, it was necessary to thaw the valve by heating it. Oftentimes, an operator would, in attempting to open the valve, apply excessive turning torque to the valve handwheel and thus damage the valve. The system would then have to be shut down and the valve repaired or replaced.
Valves are known which make both a face-to-face seal between a pair of metal seats and a so-called seatless seal with the latter being mechanically compressed in axial direction so as to cause its deformable seal member to expand radially into sealing engagement with the bore of the valve housing and the valve member. In some known valves, this deformable seal is compressed by a lost motion arrangement after the valve seats have seated. In order to fully compress this seatless seal, it is necessary for the operator to continue turning the valve handwheel after the valve has apparently "bottomed out" (i.e., after the metal seats have mated). It has been found that operators often failed to continue turning the handwheel after it "bottomed out" and thus the seatless seal was often inadequately compressed. Reference may be made to such U.S. patents as U.S. Pat. Nos. 2,720,219, 2,839,265 and 3,211,419 which disclose valves similar to the above-mentioned valves having a lost-motion connection for mechanically compressing the "seatless" seal. In other known valves, the "seatless" seal is axially compressed by the fluid pressure within the valve. These prior pressure-actuated seatless seals were not, however, well suited to high temperature, high pressure service. Reference may also be made to such U.S. patents as U.S. Pat. Nos. 763,208 and 2,114,789 which disclose valves having pressure-actuated seals.